Most speedometers measuring translational speed of a moving body either are not stand-alone or do not directly measure the speed. A speedometer that is not stand-alone uses contact information from outside of the moving body. For example, the speedometer in a car only works when the wheel of the car contacts with the ground, the Pitot tube of an airplane only works when the Pitot tube probes the surrounding air, the sonar of a submarine only works when sound wave reflects from some reference objects, and the GPS receiver only works when it receives the signal from the GPS satellites. A speedometer that is not directly measuring the moving speed calculates the speed based on other measurable information. For example, an accelerometer in a navigation system measures the translational acceleration and the translational speed is determined by integrating the accelerometer output with an initial speed.
A patent (U.S. Pat. No. 6,813,006) has invented a stand-alone speedometer directly measuring the translational speed. The present invention provides a new method and a new speedometer that utilizes two spaced laser beams emitted from a laser and measures their travel-time difference.